1. Field of the Invention
The present invention relates to an image forming apparatus such as a color printer or a color copying machine which improves color reproducibility and a control method therefor.
2. Description of the Related Art
Recently, electrophotographic image forming apparatuses and the like such as color printers and color copying machines have been required to improve image quality. If, however, components of an image forming apparatus vary in operation due to environmental changes or long use, the obtained images vary in density. An electrophotographic image forming apparatus, in particular, varies in density due to even slight environmental fluctuation, which may lead to a deterioration in color balance. For this reason, the image forming apparatus needs to include a means for keeping density and tone characteristics constant.
Japanese Patent Laid-Open No. 2003-084532 discloses a correction means for correcting process conditions such as exposure amount and developing bias and a lookup table (LUT) in accordance with temperature and humidity with respect to the toner of each color.
The correction means forms toner patches on a printing material by using toners of the respective colors so as to obtain constant density or tone characteristics even with variations in the components of the apparatus. The correction means then detects the density or chromaticity of each toner patch by using a sensor. Finally, the correction means performs density calibration control by applying feedback to process conditions such as exposure amount and developing bias in accordance with the detection result. This stabilizes the density or chromaticity of the formed image.
However, the following problems arise in the image forming apparatus disclosed in Japanese Patent Laid-Open No. 2003-084532. In general, when adjusting the color balances, the image forming apparatus preferably forms toner patches by using the same image formation conditions as those at the time of normal image formation. In addition, the image forming apparatus preferably detects toner patches at the same feeding speed as that at the time of normal image formation.
In general, however, the image forming apparatus changes the feeding speed of a printing material in accordance with the type of printing material (e.g., plain paper or thick paper). For example, the feeding speed of a printing material such as thick paper is set lower than that of plain paper. Color balance adjustment using a printing material with a low feeding speed requires more time than color balance adjustment using plain paper.